The present invention relates to magnetic iron oxide particles and a magnetic toner using the magnetic iron oxide particles. More particularly, the present invention relates to magnetic iron oxide particles which can be present in the form of small agglomerated particles and have an excellent dispersibility, and a magnetic toner using the magnetic iron oxide particles.
As one of conventional electrostatic latent image-developing methods, there has been widely known and generally employed a so-called one component-system magnetic toner development method in which composite particles prepared by mixing and dispersing black magnetic iron oxide particles such as magnetite particles in a resin are used as a developer without a carrier.
With the recent tendency of high-speed copying and high-image quality for laser beam printers or digital copying machines, it has been strongly required to improve properties of a magnetic toner used as a developer therefor. For this purpose, the magnetic toner has been strongly required to exhibit an improved electrification performance.
The electrification performance of the magnetic toner has a close relationship with a dispersibility of the magnetic iron oxide particles contained therein. For this reason, the magnetic iron oxide particles have been strongly required to exhibit an excellent dispersibility.
That is, the magnetic toner has been produced by mixing magnetic particles in a resin, for example, an aromatic vinyl-based resin such as polymers composed of styrene and vinyl toluene, acrylic resins such as polymers composed of acrylic acid and methacrylic acid, and copolymer resins thereof. Therefore, the magnetic particles have been required to exhibit an excellent dispersibility in these resins.
The magnetic iron oxide particles having an excellent dispersibility mean particles which are excellent in easiness of dispersion in resins such that when the magnetic iron oxide particles are kneaded with the resins to obtain a resin molded product, agglomerated particles composed of the magnetic iron oxide particles can be deaggregated as fine as possible until being lowered in size into primary particles thereof or fine particles substantially identical to the primary particles thereof, or particles capable of completing such a kneading treatment for a short period of time.
However, the magnetic iron oxide particles are usually firmly agglomerated together to form hard large particles. For this reason, in the case where the resin molded product is produced by kneading the magnetic iron oxide particles with the resins, there tend to arise problems such as a prolonged dispersing time for deaggregating the agglomerated particles into individual small particles as well as increase in dispersing power of a kneader or a dispersing apparatus used therefor.
In addition, the magnetic iron oxide particles themselves tend to be magnetically agglomerated together due to fine particles. Therefore, when the magnetic iron oxide particles are kneaded with the resins upon production of a magnetic toner, it may be difficult to mix the magnetic iron oxide particles in the resins with a sufficient dispersibility. As a result, the magnetic iron oxide particles may fail to be dispersed in a state deaggregated into primary particles and, therefore, tend to be frequently still present in the form of the agglomerated particles.
In the case where a magnetic toner having a desired particle size is produced by pulverizing a kneaded material in which the magnetic iron oxide particles are not sufficiently dispersed, there tend to be caused problems such as liberation of magnetic particles from the agglomerated particles upon the pulverization as well as non-uniform content of the magnetic particles in individual magnetic toner particles, thereby failing to obtain a magnetic toner having an excellent electrification stability.
Conventionally, there have been conducted many attempts for improving a dispersibility of the magnetic iron oxide particles by pulverizing the particles using various pulverizers (Japanese Patent Application Laid-Open (KOKAI) Nos. 2-80(1990), 6-67453(1994), 8-259238(1996) and 2003-192351).
At present, it has been most strongly demanded to provide magnetic iron oxide particles for a magnetic toner which exhibit not only a good compatibility with resins used for the magnetic toner, in particular, aromatic vinyl-based resins, acrylic resins and copolymer resins thereof, but also a good dispersibility in these resins. However, such magnetic iron oxide particles fully satisfying these requirements have not been obtained until now.
That is, in Japanese Patent Application Laid-Open (KOKAI) No. 2-80(1990), there is described a method of pulverizing magnetic iron oxide particles using a Fred mill. However, in this method, the obtained magnetic iron oxide particles are still insufficient in dispersibility.
Also, in Japanese Patent Application Laid-Open (KOKAI) No. 6-67453(1994), there is described a method of pulverizing magnetic iron oxide particles using the combination of a wheel-type kneader and an impact-type pulverizer. However, in this method, the obtained magnetic iron oxide particles also are still insufficient in dispersibility.
Further, in Japanese Patent Application Laid-Open (KOKAI) No. 8-259238(1996), there is described a method of treating magnetic particles using a container-fixed horizontal double-axis type kneader to improve a dispersibility thereof. However, when aromatic vinyl-based resins, acrylic resins or copolymer resins of constituting monomers thereof are used as a resin for a magnetic toner, the magnetic particles fail to be sufficiently improved in dispersibility therein and, therefore, the magnetic toner produced therefrom is still insufficient in electrification stability.
In addition, in Japanese Patent Application Laid-Open (KOKAI) No. 2003-192351, there are described octahedral magnetic iron oxide particles having a number-average particle diameter of 0.2 to 1.0 μm as measured by a laser diffraction scattering-type particle size distribution method. However, since a ratio of the particle diameter as observed by SEM to the number-average particle diameter as measured by the laser diffraction scattering-type particle size distribution method is as large as not less than 3, agglomerated particles of the magnetic iron oxide particles have a too large particle size, so that the magnetic iron oxide particles fail to show an excellent dispersibility.
As a result of the present inventors' earnest studies, it has been found that by mixing an aqueous ferrous salt solution and an aqueous alkali hydroxide solution with each other for subjecting to the neutralization treatment, subjecting the mixed solution to oxidation treatment, and subjecting a slurry obtained after completion of the oxidation reaction to filtration and water-washing and then successively to wet-pulverization and drying in fluidized bed, the resultant magnetic iron oxide particles can be present in the form of small agglomerated particles (secondary particles), and can exhibit an excellent dispersibility. The present invention has been attained on the basis of this finding.